Hervé Fritz

Behavioral adjustments of African herbivores to predation risk by lions: spatiotemporal variations influence habitat use.

Predators may influence their prey populations not only through direct lethal effects, but also through indirect behavioral changes. Here, we combined spatiotemporal fine-scale data from GPS radio collars on lions with habitat use information on 11 African herbivores in Hwange National Park (Zimbabwe) to test whether the risk of predation by lions influenced the distribution of herbivores in the landscape. Effects of long-term risk of predation (likelihood of lion presence calculated over four months) and short-term risk of predation (actual presence of lions in the vicinity in the preceding 24 hours) were contrasted. The long-term risk of predation by lions appeared to influence the distributions of all browsers across the landscape, but not of grazers. This result strongly suggests that browsers and grazers, which face different ecological constraints, are influenced at different spatial and temporal scales in the variation of the risk of predation by lions. The results also show that all herbivores tend to use more open habitats preferentially when lions are in their vicinity, probably an effective anti-predator behavior against such an ambush predator. Behaviorally induced effects of lions may therefore contribute significantly to structuring African herbivore communities, and hence possibly their effects on savanna ecosystems.

In search of a real definition of the biological invasion phenomenon itself

The many qualifying terms attributed to invasive species reveal the lack of precision surrounding the notion of biological invasion itself. In spite of several proposed definitions, some basic disagreements persist concerning characterization of the phenomenon. These primarily arise from the lack of pertinence of both of the main current criteria—the geographic (or biogeographic) criterion and the impact criterion—to what is really intended by “invasion.” Faced with this situation, it seems preferable to adopt an ontological approach allowing a return to the basic principles of the elaboration of a definition. Starting with the nature of the phenomenon itself (i.e., its essence), we try to elucidate the notion of biological invasion and we suggest a general definition compatible with most of the ideas already expressed.

On the Carrying Capacity for Large Ungulates of African Savanna Ecosystems

The effect of pastoral management on the standing crop biomass of large ungulates, an index of carrying capacity, is analysed at a regional level by using data compiled from published sources covering east and south Africa. The effect of primary production is controlled for by using two environmental factors, rainfall and soil nutrient availability, and the effect of species richness of the ungulate community is evaluated. The results confirm the dominant effect of rainfall, and demonstrate that soil nutrient levels also strongly affect the biomass of large ungulate communities. For a given level of rainfall, on rich soils (high nutrient availability) the biomass of large ungulates is about 20 times as great as on poor soils with low nutrient availability. The model based on rainfall and soil nutrient classes accounts for 87% of the variance in large ungulate biomass. Pastoral and natural ecosystems do not differ significantly in large ungulate biomass; there is therefore currently no evidence that extensive pastoral management increases carrying capacity for large ungulates above the levels observed in natural communities. Species richness, a measure of biodiversity, had a significant but very small effect on the biomass-rainfall relation in the complete data set. When analysed by pastoral management, the effect of this factor was significant for the set of natural ecosystems only. Pastoral management and species richness may therefore have compensatory effects. These results suggest that the carrying capacity is limited at the community, rather than the species, level.

Megaherbivores influence trophic guilds structure in African ungulate communities

We used a data set of ungulate censuses from 31 natural ecosystems from East and Southern Africa to test two hypotheses: (1) megaherbivores should dominate ungulate communities in ecosystems with high rainfall and low soil nutrient status because of their ability to survive on poor quality food resources, and (2) the abundance of megaherbivores affects the abundance of the mesoherbivores, distinguishing the different feeding guilds: mesograzers, mesobrowsers and mesomixed feeders. Two axes of a multivariate analysis (77% of the variance) discriminated the sites well, the first separating sites dominated by megaherbivores from those dominated by mesoherbivores, and the second representing a gradient between mesograzers and mesobrowsers. Our analysis shows (1) that megaherbivores can be considered to be a separate trophic guild and (2) that mesograzers and mesobrowsers respond differently to variation in their trophic environments. The metabolic biomass density of megaherbivores increased with annual rainfall, but was not related to soil nutrient status, and as predicted, megaherbivores comprised a larger proportion of the biomass of ungulate communities in ecosystems with high rainfall and low nutrient soils. The metabolic biomass density of mesoherbivores increased with rainfall and soil nutrient status. Within the mesoherbivores, the metabolic biomass density of mesograzers showed the same trend, and seemed unaffected by megaherbivores. Conversely, mesobrowsers and mesomixed feeders appeared to be unaffected by rainfall or soil nutrient status, but mesomixed feeders declined when megaherbivores were abundant. This suggests that megaherbivores may compete with the mesomixed-feeder species for food or they may alter the vegetation communities unfavourably. A similar analysis using elephants alone instead of megaherbivores as a group showed that both mesobrowsers and mesomixed feeders were affected significantly by elephant, which is consistent with the fact that most of the effect of megaherbivores on browse resources or woodland habitat is due to elephants. This study shows that the different trophic guilds within African ungulate communities react differently to environmental factors (rain and soil), and that megaherbivores, and particularly elephants, appear to compete with mesomixed feeders and mesobrowsers. These results are relevant for the understanding of the functioning of African ungulate communities and call for further testing with longitudinal data.

Which functional processes control the short-term effect of grazing on net primary production in grasslands?

Grazing has traditionally been viewed as detrimental to plant growth, but it has been proposed that under certain conditions, grazing may lead to compensatory or overcompensatory growth. However, comprehensive information on the relative role of the main functional processes controlling the response of net primary production (NPP) to grazing is still lacking. In this study, a modelling approach was used to quantify the relative importance of key functional processes in the response of annual canopy NPP to grazing for a West African humid grassland. The PEPSEE-grass model, which represents radiation absorption, NPP, water balance and carbon allocation, was used to compute total and aboveground NPP in response to grazing pressure. Representations of grazing and mineral nitrogen input to the canopy were simplified to focus on the vegetation processes implemented and their relative importance. Simulations were performed using a constant or resource-driven root/shoot allocation coefficient, and dependence or independence of conversion efficiency of absorbed light into dry matter on nitrogen availability. There were three main results. Firstly, the response of NPP to grazing intensity emerged as a complex result of both positive and negative, and direct and indirect effects of biomass removal on light absorption efficiency, soil water availability, grass nitrogen status and productivity, and root/shoot allocation pattern. Secondly, overcompensation was observed for aboveground NPP when assuming a nitrogen-dependent conversion efficiency and a resource-driven root/shoot allocation. Thirdly, the response of NPP to grazing was mainly controlled by the effect of plant nitrogen status on conversion efficiency and by the root/shoot allocation pattern, while the effects of improved water status and reduced light absorption were secondary.

Does the risk of encountering lions influence African herbivore behaviour at waterholes

A central question in the study of predator–prey relationships is to what extent prey behaviour is determined by avoidance of predators. Here, we test whether the long-term risk of encountering lions and the presence of lions in the vicinity influence the behaviour of large African herbivores at waterholes through avoidance of high-risk areas, increases in group size, changes in temporal niche or changes in the time spent in waterhole areas. In Hwange National Park, Zimbabwe, we monitored waterholes to study the behaviour of nine herbivore species under different risks of encountering lions. We radio-collared 26 lions in the study area which provided the opportunity to monitor whether lions were present during observation sessions and to map longer-term seasonal landscapes of risk of encountering lions. Our results show that the preferred prey species for lions (buffalo, kudu and giraffe) avoided risky waterholes. Group size increased as encounter risk increased for only two species (wildebeest and zebra), but this effect was not strong. Interestingly, buffalo avoided the hours of the day which are dangerous when the long-term and short-term risks of encountering lions were high, and all species showed avoidance of waterhole use at night times when lions were in the vicinity. This illustrates well how prey can make temporal adjustments to avoid dangerous periods coinciding with predator hunting. Additionally, many herbivores spent more time accessing water to drink when the long- and short-term risks of encountering lions were high, and they showed longer potential drinking time when the long-term risk of encountering lions was high, suggesting higher levels of vigilance. This study illustrates the diversity of behavioural adjustments to the risk of encountering a predator and how prey respond differently to temporal variations in this risk.

Spatial patterns of the NDVI–rainfall relationship at the seasonal and interannual time scales in an African savanna

Climate change is predicted to affect both the mean annual rainfall and its seasonal distribution over the African continent. Understanding their respective influences on primary production, an ecosystems key feature, is therefore a major challenge for rangeland ecologists. We have investigated the change in intra‐ and interannual Normalized Difference Vegetation Index (NDVI) in relation to rainfall in Hwange National Park, Zimbabwe. Two distinct NDVI time series were built using NOAA/AVHRR data for the period 1982–2002. Long‐term monthly means described the change in seasonal NDVI, whereas annually integrated NDVI related to year‐to‐year fluctuations. The rainfall–NDVI relationship was stronger along the seasonal course [with a lag of 1 month, Kendalls tau (τ) = 0.879] than when studied interannually (τ = 0.476). Principal component analysis (PCA) demonstrated that spatial patterns of the NDVI fluctuations differed when studied interannually or during the seasonal course. Field features such as topography or vegetation composition influenced seasonal NDVI values whereas only rainfall distribution played a role at the interannual time scale. Our results show that rainfall controls on primary production and their mitigation differ between time scales, and these findings bring insights on the future response of savannas to climate change.

Invasive species can also be native

In a recent review in TREE, Wilson et al. [1] aim to link success of biological invasions and the dispersal pathways of species. The authors emphasize the characteristics of human-mediated extra-range dispersal pathways (i.e. rapidity, high propagule pressure and high genetic diversity of the introduced population) in order to argue that biological invasions are, above all, a biogeographical phenomenon of unprecedented magnitude and, thus, that it would be better to separate biological invasions from ‘considerations of ecological dominance’ [1].

The importance of protected areas as nocturnal feeding grounds for dabbling ducks wintering in western France

We studied the diurnal and nocturnal habitat use of wintering dabbling ducks (Anas spp.) in two protected areas of an internationally important winter quarter in western France. The waterbodies of the reserves are heavily used by ducks during daylight hours, and 3–55% of these birds used the reserves at night: >50% of shoveler (A. clypeata), 20% of granivorous ducks (mallard A. platyrhynchos, teal A. crecca and pintail A. acuta), and lower numbers of herbivores (wigeon A. penelope and gadwall A. strepera). Radio-tracking showed that some ducks used the reserves by day and by night, and that some of them may switch from one protected site to another: radio-tagged birds were located in one of the two protected areas for 76% of the days and 81% of the nights they were sought, with granivores switching from waterbodies to wet grasslands within a reserve between the two periods. Such resident individuals may be ‘experienced’ wintering ducks, avoiding surrounding unprotected feeding habitats at night, while birds that leave the reserves at night may be subdominants and/or ‘naive’ individuals from a transient migratory sub-population. This study suggests that management of nature reserves should combine day-roosts with significant areas of nocturnal feeding grounds, since in protected areas both habitats may be successively used by wintering dabbling ducks across the 24-h cycle.

Individual vigilance of African herbivores while drinking: the role of immediate predation risk and context

Perceived risk of predation is considered to influence individual prey vigilance, but little is known about the extent to which the context (e.g. distance to cover, group size) and the immediate presence of predators determine individual prey vigilance. We combined behavioural data on individual vigilance of herbivores at waterholes with fine-scale spatiotemporal data from 27 African lions, Panthera leo, fitted with GPS radiocollars to test whether individual prey vigilance increases when predators are in the vicinity and whether this relationship is influenced by group size and presence of other herbivores. The study was conducted on giraffe, Giraffa camelopardalis, greater kudu, Tragelaphus strepsiceros, and plains zebra, Equus quagga, in Hwange National Park, Zimbabwe, between mid-September and mid-November 2008. Kudu was the only species to adjust its vigilance level significantly in the presence of lions, arguably owing to its higher contribution to lion diet in Hwange. Kudu devoted a significantly higher proportion of time to vigilance when lions were in the vicinity. Furthermore, the proportion of time spent on individual vigilance decreased as group size increased during the approach and drinking phases, particularly when lions were in the vicinity. Finally, the presence of other herbivores at the waterhole enabled kudu to spend less time vigilant while drinking. These adjustments involved changes in the length of vigilance bouts, but not in their frequency. Our study suggests that the interplay between the context (group size, presence of other prey) and the immediate presence of predators determines the level of vigilance in prey.